Optical amplifiers serve to amplify light signals which are transmitted over a transmission path, in particular a fiber optical waveguide (optical fiber), and are weakened by attenuation so that they must be amplified.
An optical amplifier with a length of fiber that contains a laser-active substance for amplifying light signals, which is coupled to at least one semiconductor laser serving as a pump light source, which is activated by an operating current, is known e.g. from: M. Yoshida et al: "Development of Compact Er.sup.3+ -doped Fiber Amplifiers for Practical Applications", Technical Digest on Optical Amplifiers and their Applications 1990 (Optical Society of America, Washington, DC 1990), Volume 13, pages 282 to 285. It is a fiber amplifier with an erbium-doped length of fiber. Erbium is a laser-active substance, which is excited by a semiconductor laser serving as a pump laser. In this instance, the semiconductor laser emits light at a wavelength of .lambda.=1480 m. This light is introduced into the doped length of fiber by an optical coupler. Direct current is used as the operating current to control the semiconductor laser.
Instead of a single pump laser, as shown in FIG. 1a of said reference, two pump lasers as shown in FIG. 1c thereof can also be provided, whose pump light is coupled into each end of the length of fiber.
A disadvantage of this optical amplifier is that the pump light produced by the semiconductor laser is reflected back into the semiconductor laser at all points along its propagation path where refractive index skipping occurs (e.g. in the area of the optical coupler or in the transition between the optical fiber and the erbium-doped length of fiber). This degrades the efficiency of the semiconductor laser; the reflected light can cause amplitude fluctuations in the pump light emitted by the semiconductor laser, because the laser-active layer in the semiconductor laser acts as a laser resonator, together with part of the transmission path (i.e. up to the area with the refractive index skip). This changes the emission frequency of the semiconductor laser, on the one hand, and subjects the amplitude of the pump light to oscillations, on the other. In particular, the amplifying effect of the length of fiber is temporarily cancelled when the oscillations occur in the low frequency range, e.g. in the kilohertz range; this can interrupt the optical transmission, under certain circumstances.
In the event two pump lasers are present (semiconductor lasers in each instance), the described problem exists for each of the two pump lasers. Furthermore, in that case there is the problem of residual pump light from each pump laser entering the opposite pump laser, which causes optical instabilities, which in turn lead to the above described problems of fluctuations in amplification. For that reason, optical isolators are presently used before the pump lasers.